Imaging for various applications, including space applications (e.g., satellite servicing) and machine vision applications (e.g., quality control and matching of paint colors), is becoming more important.
Conventional color video cameras use only three colors of light to produce information about the three colors perceived by human vision: red, green, and blue. This is typically done by using three focal planes, which is wasteful, or by filtering only one of the three colors into each pixel of the focal plane, which results in information loss.
However, a computer vision system can use many more “colors”, as such a system can use an arbitrarily large number of wavelength-classes. Furthermore, instrument-assisted spectral imaging can extend the range to include not only the visible band but also the ultraviolet and infrared regions of the spectrum invisible to humans. Accordingly, state of the art imaging is beginning to include multi-spectral data. To produce multi-spectral data, state of the art methods generally send an entire 2-D light stream (usually having a circular cross-section) through a color filtering device that typically includes large, delicate, and expensive hardware for providing filters that can filter the entire 2-D light beam.
Modern multi-spectral computer vision systems may use electronically tunable filter (ETF) technology, by which an entire beam of light passing through a camera is filtered into a narrow band of color by an ETF (e.g., accousto-optical, Fabry-Perot, liquid crystal). ETFs can generally fit a large 2-D beam through their clear apertures, but they share the qualities of complexity, expense, fragility, specialized electronic power requirements, and use of materials that may be sensitive to radiation.
Simpler filters, such as variable filters (e.g., linear or circular variable filters), are inexpensive and robust but have not been suited for image filtering as they typically filter a 1-D beam of light having a narrow cross-section (usually produced in scientific instruments that do not create 2-D images). If such variable filters were used to filter an entire 2-D light stream (e.g., having a circular cross-section), up to about 90% of the incoming light would be wasted.
Thus, a robust system and method for spectral filtering of images with improved size, power, and weight characteristics is highly desirable.